Recently, there is a growing spread of personal network appliances along with the development of cellular telephony networks and wireless LANs. In such a situation, considerations have being made on the ubiquitous network that networks everything at the around in order to realize network access at any time and everywhere. For example, it is described in the document of a report “Study Group on the Future Overview of Ubiquitous Network Technology”—toward realizing a network with anything at everywhere, Jun. 11 2002, Information & Communications Policy Bureau of Ministry of General Affairs, Press Release, the Internet <URL: http://www.soumu.go.jp/s-news/2002/020611#4.html>. Conventionally, various mobile communication systems have been proposed in order to realize such a ubiquitous network.
FIG. 41 is a diagram showing a configuration of a mobile communication system described in JP-A-2000-13823.
In FIG. 41, a mobile communication unit 4101 is allowed to connect with a line control 4102a through a wireless base station 4103, to thereby obtain information on the type of the Internet 4104, a home intranet 4105, a public-switched telephone network (PSTN), an ISDN and a PHS 4106 as an external network usable from the line control 3102a. The mobile communication unit 4101 selects an optimal route on the basis of the information and makes a connection request to the line control 4102a and communicates with an external network selected. When the mobile communication unit 4101 moves and the communication zone is changed, the mobile communication unit 4101 makes notification to a new one of line control 4102b and selects an external network in the similar manner.
Meanwhile, FIG. 42 is a figure showing a configuration of a mobile communication system described in JP-T-2003-514442.
In FIG. 42, a mobile communication device 4202 is allowed to selectively access a network 4203 through one or more access-network terminal device 4201. An access-network terminal device 4202 is decided on the basis of a comparison between a capability of the access-network terminal device 4202 and a preferable capability. During communication with the network 4203, the mobile communication device 4202 continuously detects whether or not a new access-network terminal device 4201 can be utilized and compares the capability thereof with the access-network terminal device 4202 currently in connection. In a case the new one is more preferable, switchover is made to it.
However, in the mobile communication system described in JP-T-2000-13823, the mobile communication unit 4101 notifies an attribute from the mobile communication unit to the line control 4102. Otherwise, unless there is an inquiry, the information about the external network cannot be newly obtained, making it difficult to swiftly cope with a change in usability situation of the external network.
Meanwhile, in the mobile communication system described in JP-T-2003-514442, the mobile communication unit 4202 periodically searches for another available mobile routers 4201 in order to use the optimal one of mobile routers 4201 even after connection. In a case there comes a suitable one, connection is reset to that mobile router 4201. Consequently, because the mobile communication unit 4202 regularly searches for an optimal one of mobile router 4201, there is a heavy burden on the mobile communication unit 4202. Furthermore, there is a problem of frequent exchange of control signals in order to acquire the information of the mobile router 4201.